Recent air pollution control technologies and wastewater treatments have altered residual byproduct streams at coal-fired power plants and created a need for new applications for beneficial use or safe disposal of these byproduct streams. Fly ash from coal-fired power plants, for example, has been extensively employed as a substitute for Portland cement in concrete. New regulations and air pollution control technologies may fundamentally alter the bulk material properties of fly ash to create entirely new byproduct categories and thereby reduce the amount of fly ash byproduct that can be beneficially used for concrete. Sorbents, such as activated carbon, which may be injected into power plant flue gas to adsorb vapor-phase mercury, and dry sorbent injection (“DSI”) chemicals injected into the duct for acid gas control will be admixed with combustion fly ash and the resulting new materials will present entirely different aggregate characteristics including particle size, density, chemical makeup, pH, heavy metals mobility, reactivity with concrete additives and others. In particular, fly ash with activated carbon above minimal levels is not suitable for concrete admixture due to reaction of the spent activated carbon with concrete air entrainment chemicals. These changes create a need for alternative byproduct uses for fly ash and in certain situations new fly ash disposal methods.
New regulations in the United States, namely Mercury and Air Toxics Standards or MATS, require control of acid gas discharge from new and existing power plants. The regulations will require control of HCl to as high as 97% removal. Many plants will inject alkaline sorbents, such as hydrated lime, sodium bicarbonate or trona, into the duct as acid gas sorbents. The unreacted alkaline sorbents along with the products of reaction will be intermixed with the fly ash. Variable content of sodium and calcium sorbents will make use of this fly ash for concrete difficult. Sodium sorbents additionally may increase concentrations of arsenic, selenium and chlorides in the fly ash, creating leachability concerns if the ash were to be landfilled. The arsenic, selenium, SO2, SO3, and HCl that had previously been emitted to the atmosphere as a stack gas will now be shifted to the solid waste stream as a result of tighter air pollution control. As a result of these changes, there is an emerging need for large-scale beneficial uses for fly ash/dry alkaline sorbent byproduct stream other than concrete admixture.
Waste water discharge limits for a variety of pollutants from coal-fired plant flue gas desulfurization scrubbers (“FGD”) are being revised to reduce harmful pollution to surface waterways. It is expected that individual point discharges, rather than aggregate plant discharge, will now be monitored and controlled, thereby creating a need to minimize point source water discharge in order to comply with the limits. New discharge limits for selenium in waste water are especially challenging due to the very soluble selenates, SeO4, that cannot be controlled by traditional chemical or physical water treatment. Emerging technologies such as bio-reactors are under development to allow polishing the water to meet new Effluent Limit Guidelines, but these are expensive and unproven.
There is a growing awareness and acceptance that flue gas desulfurization (“FGD”) scrubber wastewater treatment and treated water discharge to surface water bodies may no longer be viable. Entirely new processes around Zero Liquid Discharge (“ZLD”) of waste water to surface water bodies are being developed. Some of these treatment processes will dewater solids from FGD wastewater by spray dryer or other evaporators. This will eliminate water discharge but produce an entirely new and potentially hazardous residual sludge for disposal as a solid waste stream. These solids will have high concentrations of chlorine, bromine and soluble trace metals, in particular selenium.
Powder River Basin fly ash is high in calcium, pozzolanic and can be used as a Portland cement substitute. It is known in the art to form a cementitious material with fly ash and Portland cement that can be added to hazardous solids as a binder and encapsulant. Such mixtures after hardening for a month to final strength will significantly reduce hazardous metal leachability (mobility). However, Portland cement is expensive. Additional greenhouse gas CO2 is generated during limestone calcining to make Portland cement. The concrete reactions are relatively slow and strength is poor if high percentages of filler or diluents such as hazardous wastes or activated carbon in fly ash are incorporated into the aggregate.
There are also known geopolymers that are alkali-activated binders for alumino-silicate base materials such as kaolinites, silicates, bentonite and fly ash, as examples. These have also been tested and are known for Solidification/Stabilization (“S/S”) of hazardous solids. However, many such geopolymers require some heating or pressure or both to form the polymers. The geopolymers are typically very caustic NaOH or KOH mixtures with handling difficulties. Most existing geopolymers are also sensitive to the incorporation of non-polymer fillers that can disrupt the structure and bonding.